1. Field of the Invention
This invention relates to a picture signal coding method and decoding method and a picture signal recording medium suitable for use to record and reproduce a coded moving picture signal onto and from a recording medium such as, for example, a compact disk or a hard disk.
2. Description of the Related Art
When it is tried to record and reproduce moving picture data, a recording medium having a very high continuous transmission rate is required since they involve a very great amount of information. At present, a video signal, for example, of the NTSC television system, is recorded onto and reproduced from a magnetic tape or an optical disk.
If it is tried to record a video signal for a long time onto a recording medium which has a comparatively small size and has a comparatively information recording capacity, then means is essentially required for coding and recording the video signal in a high efficiency and decoding a read signal of the thus recorded video signal in a high efficiency. In order to meet the requirement, several high efficiency coding systems which make use of correlations of a video signal have been proposed, and one of the systems is the MPEG (Moving Picture Experts Group) system. Details of the MPEG system are disclosed in AVC-400 TEST MODEL 3, ISO-IEC/JTC1/SC29/WG11, November, 1992. Another system wherein a coded signal is divided into high priority data and low priority data and then the high priority data and the low priority data are transmitted on different carriers from each other is investigated by the ATV of the United States.
In the MPEG system, a difference between picture frames of a video signal is taken first to decrease the redundancy in the time axis direction, and then an orthogonal transformation technique such as discrete cosine transform (DCT) is used to decrease the redundancy in the space axis direction. A video signal is coded efficiently in this manner and recorded onto a predetermined recording medium. Recording of a video signal coded in a high efficiency is disclosed, for example, in U.S. Pat. No. 5,140,437, U.S. Pat. No. 5,040,061, Japanese Laid-Open Patent No. 3-129979 and Japanese Laid-Open Patent No. 3-78380.
On the other hand, when a recording medium on which a video signal coded in a high efficiency is recorded in this manner is reproduced, the reproduction signal is transformed by inverse orthogonal transformation to decode it in a high efficiency to reproduce the video signal.
By the way, when a recording medium on which a video signal coded in a high efficiency is recorded in this manner is to be reproduced at a high speed (picture search), decoding is performed after each several frames and the thus decoded signal is outputted at a rate equal to that in ordinary reproduction.
However, in a coding method according to the MPEG system described above, coding is performed based on prediction of motion between frames. Consequently, some frames (P-pictures or B-pictures) cannot be decoded without a coded picture of another frame in the past or in the future with respect to the frame at present. Accordingly, it is not always possible to arbitrarily select frames of a picture signal to reproduce the picture signal at a high speed.
If only frames (I-pictures) which can be accessed and decoded directly are reproduced, then high speed reproduction of the picture signal is possible without waiting reproduction of some other frame or frames. However, the I-picture (intra-frame coded frame whose decoding is completed within the frame) is normally present by in only one frame of each ten and several frames (GOP which will be hereinafter described), and even if only such intra-frame coded frames are reproduced, only high speed reproduction with rough motion can be performed.
For example, a continuously inputted video signal is grouped into a GOP (Group of Pictures) which includes 15 frames. Then, as shown in FIG. 57, the first two frames of a GOP are processed as B-pictures (B0, B1), and then the next frame is processed as an I-picture (I2). Thereafter, coding processing is performed such that P-pictures (P5, P8, P11, P14) may be produced with two B-pictures (B3, B4, B6, B7, B9, B10, B12, B13) interposed between each two adjacent ones of them.
The data coded in this manner are transmitted in the order of I2, B0, B1, P5, B3, B4, PS, B6, B7, P11, B9, B10, P14, B12 and B13 as seen in FIG. 58. This is because, since, for example, each of the B-pictures B0 and B1 (or B3 and B4) requires, as a predictive frame, another frame I2 (P5) which appears later in time, it cannot be decoded if the frame I2 (P5) is not prepared in advance.
While high speed reproduction is possible if coding processing is performed at a high speed for all of the frames B0 to P14, this is substantially impossible due to a restriction in software to raise the decoding processing speed by several times.
Therefore, if each I-picture and each P-picture for which another frame preceding in time is used as a predictive frame are reproduced but no B-picture is reproduced as seen from FIG. 58, then finer high speed reproduction can be performed. The high speed reproduction method is disclosed in U.S. patent application Ser. No. 07/981,178 now U.S. Pat. No. 5,305,113. In this instance, however, so much time is required to effect track jumping to search a next P- or I-picture after another I- or P-picture has been reproduced that the high speed reproduction picture may possibly be interrupted. Further, a method wherein some of P-pictures and B-pictures are used in addition to I-pictures for high speed reproduction is disclosed in U.S. patent application Ser. No. 125,685 now U.S. Pat. No. 5,504,585. Further, a technique wherein an entry packet indicative of an access point upon high speed reproduction is inserted into a bit stream is disclosed in U.S. patent application Ser. No. 125,573 no U.S. Pat. No. 5,455,684.
Further, an I-picture involves a greater amount of data than P- and B-pictures and consequently data of I-pictures are read out, upon high speed reproduction, at a high frequency than upon ordinary reproduction. Consequently, there is a problem to be solved in that failure to read data of an I-picture sometimes occurs.